Activin A and Activin B: TGF-β Superfamily Ligands Regulating Erythropoiesis, Vascular Remodeling, and Pulmonary Arterial Hypertension with Sotatercept Approval in Research

# Activin A and Activin B: TGF-β Superfamily Ligands Regulating Erythropoiesis, Vascular Remodeling, and Pulmonary Arterial Hypertension with Sotatercept Approval in Research

Introduction

The transforming growth factor-β (TGF-β) superfamily is one of the largest and most pleiotropic cytokine families in vertebrate biology, encompassing TGF-βs, bone morphogenetic proteins (BMPs), growth differentiation factors (GDFs), activins, inhibins, and Müllerian inhibiting substance (MIS/AMH). Within this superfamily, the activins occupy a particularly complex position — originally identified as reproductive hormones regulating pituitary FSH secretion, they have since been found to modulate erythropoiesis, inflammation, bone homeostasis, metabolism, and most relevantly for current therapeutics, pulmonary vascular remodeling.

Activin A (a homodimer of two inhibin βA subunits: βAβA) and Activin B (βBβB homodimer) are the primary activin ligands, with Activin AB (βAβB heterodimer) also present physiologically. They signal through type II activin receptors (ActRIIA and ActRIIB) in complex with type I receptors (primarily ALK4 for activin A, ALK7 for activin B and AB) to phosphorylate SMAD2 and SMAD3, which complex with SMAD4 and regulate transcription.

The clinical pivot point came with the recognition that dysregulated activin/TGF-β signaling drives pulmonary arterial hypertension (PAH) — a progressive, life-threatening disease characterized by obliterative pulmonary vascular remodeling. Sotatercept, a fusion protein trap consisting of the extracellular domain of ActRIIA fused to an IgG1 Fc region, received FDA approval in March 2024 for PAH treatment — marking the first mechanistically new PAH therapy in over a decade.

Simultaneously, activin biology intersects with erythropoiesis through SMAD2/3 signaling in erythroblasts — the mechanism exploited by luspatercept (ActRIIB-Fc trap) approved for β-thalassemia and lower-risk MDS anemia. Understanding activin signaling is thus essential for researchers working across PAH, hematology, reproductive endocrinology, and metabolism.

Discovery and Early Characterization

Reproductive Endocrinology Origins

Activin was independently discovered by multiple groups in the 1980s through functional screens for factors that stimulate pituitary FSH (follicle-stimulating hormone) secretion:

Ling et al. (1986) isolated a protein from porcine follicular fluid that stimulated FSH release from pituitary cells, naming it "activin" (Nature,). Simultaneously, Vale et al. (1986) isolated a structurally related protein that inhibited FSH — naming it inhibin (Nature,).

Biochemical analysis revealed that activin and inhibin share subunit structure:

• •Inhibin A: α-subunit (INHA) + βA-subunit (INHBA) heterodimer
• •Inhibin B: α-subunit (INHA) + βB-subunit (INHBB) heterodimer
• •Activin A: βA-βA homodimer
• •Activin B: βB-βB homodimer
• •Activin AB: βA-βB heterodimer

Thus activins and inhibins share the β subunits but differ in whether they dimerize with α-subunit (inhibin, which blocks activin receptor access) or each other (activin, which activates receptors). Follistatin (FST), discovered around the same time, is a third regulatory protein that binds and neutralizes activins with high affinity.

Expansion Beyond Reproduction

Activins were found to have biological activity far beyond the pituitary-gonadal axis:

• •Erythropoiesis: activin A promotes early erythroid progenitor differentiation (Vale et al., early 1990s)
• •Inflammation: activin A is elevated in infection, trauma, and sepsis; promotes monocyte/macrophage activation
• •Nervous system: promotes neuronal survival and differentiation
• •Wound healing: promotes keratinocyte migration and repair
• •Bone: activin A inhibits osteoblast differentiation and stimulates osteoclastogenesis via RANKL

Gene Structure and Protein Biochemistry

Subunit genes:

• •INHBA (chromosome 7p15-p13): encodes the βA subunit (used in Activin A and Inhibin A)
• •INHBB (chromosome 2q33-q36): encodes the βB subunit (used in Activin B and Inhibin B)
• •INHA (chromosome 2q33-q36): encodes the α subunit (used in both inhibins)

Protein structure:

• •Activins are disulfide-linked dimers (like all TGF-β superfamily members)
• •Mature activin subunits (~13 kDa each after signal peptide and prodomain cleavage) form dimers via a C-terminal "cystine knot" motif
• •Activin A molecular weight: ~25 kDa (dimer of two ~12-13 kDa mature βA chains)
• •Prodomains (latency-associated peptides): larger than TGF-β prodomains; cleavage by proprotein convertases (furin, PACE4) during Golgi transit releases active dimer, but prodomain association may persist and modulate receptor binding
• •Follistatin binding interface: the wrist/knuckle epitopes of activin engage follistatin's three cystatin-like modules; two follistatin molecules wrap around one activin dimer in a 2:1 stoichiometry

Signaling Pathway: ActRII → ALK4/7 → SMAD2/3

Receptor Complex Assembly

Activin signaling initiates through sequential receptor binding:

1. Activin binds type II receptor: ActRIIA or ActRIIB (both serine/threonine kinases with extracellular ligand-binding domain). ActRIIB has higher affinity for most activins.

2. Type II receptor recruits type I receptor: ALK4 (ACVR1B) for activin A, ALK7 (ACVR1C) for activin B and AB; ALK5 (TβRI) for some contexts

3. Transphosphorylation: constitutively active type II kinase phosphorylates GS-box domain of type I receptor, activating its kinase

4. SMAD2/3 recruitment and phosphorylation: activated type I receptor (ALK4/7) phosphorylates the C-terminal SSXS motif of R-SMADs (SMAD2, SMAD3)

5. SMAD complex formation: phospho-SMAD2/3 complexes with SMAD4 (Co-SMAD)

6. Nuclear translocation: SMAD2/3/4 complex enters nucleus, binds SMAD-binding elements (SBEs), recruits co-activators/co-repressors → gene regulation

Key target genes of SMAD2/3:

• •In erythroblasts: suppresses late-stage erythroid maturation genes; luspatercept mechanism targets this
• •In pulmonary smooth muscle cells: promotes proliferation, anti-apoptotic programs
• •In macrophages: regulates M2 polarization, IL-10 production
• •In liver: regulates hepcidin (interestingly, SMAD2/3 partially contributes alongside SMAD1/5/8 in hepcidin regulation)

Negative Regulators

• •Follistatin (FST) and Follistatin-like 3 (FSTL3): secreted ligand traps; bind activins (especially activin A) with sub-nanomolar affinity and prevent receptor engagement
• •Inhibins: compete with activins for ActRII binding; act as competitive antagonists (require the α-subunit which cannot signal)
• •SMAD7 (I-SMAD): inhibitory SMAD induced by TGF-β/activin signaling; provides negative feedback by competiting with R-SMADs at type I receptor and recruiting SMURF2 ubiquitin ligase for receptor degradation
• •SnoN/Ski: nuclear co-repressors that inhibit SMAD2/3 transcriptional activity

SMAD-Independent Pathways

Activin/ActRII can also signal through:

• •ERK1/2 (MAPK pathway): Erk activation by activin A in some cell types
• •p38 MAPK: stress response integration
• •PI3K/AKT: survival signaling in certain contexts
• •RhoA: cytoskeletal effects in smooth muscle

Activin A in Erythropoiesis

Normal erythropoiesis proceeds through a hierarchy: hematopoietic stem cell → burst-forming unit erythroid (BFU-E) → colony-forming unit erythroid (CFU-E) → proerythroblast → basophilic → polychromatic → orthochromatic erythroblast → reticulocyte → erythrocyte.

Activin A and GDF11 act on late-stage erythroblasts through ActRIIB/ALK5 → SMAD2/3 to suppress terminal differentiation and hemoglobin synthesis. This appears physiologically appropriate at low activin levels, but in ineffective erythropoiesis conditions (β-thalassemia, MDS), dysregulated GDF11/activin A signaling traps erythroblasts in immature stages, causing massive ineffective erythropoiesis with paradoxically poor hemoglobin production despite erythroid hyperplasia.

Luspatercept (ACE-536/REBLOZYL) is an ActRIIB extracellular domain fused to IgG1-Fc (Fc trap). By sequestering GDF11, GDF8, activin A, and activin B away from ActRIIB on erythroblasts, luspatercept reduces inhibitory SMAD2/3 signaling and allows terminal erythroid differentiation to proceed — increasing hemoglobin production without EPO receptor engagement.

Luspatercept received FDA approval:

• •April 2020: β-thalassemia in adults requiring regular red blood cell transfusions (BELIEVE trial,)
• •April 2020: MDS with ring sideroblasts, low/intermediate risk (MEDALIST trial,)
• •2023: First-line MDS (expanded indication, COMMANDS trial)

Activin Signaling in Pulmonary Arterial Hypertension

PAH Pathobiology

Pulmonary arterial hypertension (PAH) is defined by mean pulmonary arterial pressure ≥20 mmHg at rest with pulmonary vascular resistance >2 Wood units. It is characterized by:

• •Progressive obliterative remodeling of small pulmonary arteries
• •Medial hypertrophy (smooth muscle cell proliferation)
• •Intimal proliferation (endothelial cell and myofibroblast)
• •Plexiform lesions (complex vascular structures in severe PAH)
• •Adventitial fibrosis
• •In-situ thrombosis

Right ventricular pressure overload from elevated pulmonary vascular resistance → RV hypertrophy → RV failure → death. Median survival without treatment historically ~2.8 years from diagnosis.

TGF-β/BMP Signaling Axis in PAH

The genetic architecture of heritable PAH is dominated by the TGF-β superfamily:

• •BMPR2 mutations (bone morphogenetic protein receptor type 2): found in ~70-80% of heritable PAH cases; heterozygous loss-of-function mutations disrupt BMP/SMAD1/5/8 anti-proliferative signaling in pulmonary artery smooth muscle cells (PASMCs)
• •ALK1 (ACVRL1) mutations: cause hereditary hemorrhagic telangiectasia (HHT) with PAH
• •ENG (endoglin) mutations: HHT type 1 with PAH
• •GDF2 (BMP9) mutations: rare PAH subtype

The normal state in pulmonary vasculature involves a balance:

• •BMP pathway (via BMPR2/SMAD1/5/8): anti-proliferative, pro-apoptotic in PASMCs
• •TGF-β/activin pathway (via ALK4/5/7/SMAD2/3): pro-proliferative, anti-apoptotic in PASMCs

In PAH, disrupted BMPR2 shifts this balance toward TGF-β/activin dominance → PASMC proliferation, resistance to apoptosis → arterial wall thickening → increased pulmonary vascular resistance.

Activin A and Activin B in PAH

Beyond BMPR2 disruption, elevated activin A and activin B levels have been documented in PAH:

• •Serum activin A and activin B are elevated in PAH patients vs. controls
• •Activin A and GDF11 promote PASMC proliferation via SMAD2/3 in vitro
• •Pulmonary artery endothelial cells from PAH patients produce excess activin A
• •Macrophages in PAH adventitia secrete activin A, potentially sustaining local inflammatory remodeling
• •Activin B (through ALK7) may have somewhat distinct effects on RV cardiomyocyte function

Sotatercept: ActRIIA-Fc Ligand Trap

Sotatercept (ACE-011) was originally developed as an anemia treatment (same concept as luspatercept but using ActRIIA rather than ActRIIB extracellular domain). Early trials in multiple myeloma patients noted unexpectedly large increases in hemoglobin — and serendipitously also documented reduction in pulmonary artery pressure in one patient, pivoting the program.

Sotatercept binds and sequesters multiple TGF-β superfamily ligands including:

• •Activin A
• •Activin B
• •GDF11
• •GDF8 (myostatin)
• •BMP5, BMP6, BMP7

By trapping these ligands, sotatercept reduces SMAD2/3 signaling in pulmonary vascular cells, rebalancing the BMP/TGF-β axis toward SMAD1/5/8 anti-proliferative signaling.

STELLAR Trial (Phase III PAH): Hoeper et al. (2023, New England Journal of Medicine,) enrolled 163 patients with PAH (WHO Group 1) on background therapy, randomizing to sotatercept 0.7 mg/kg SC q3w vs. placebo for 24 weeks.

Primary endpoint: 6-minute walk distance (6MWD) improvement

• •Sotatercept: +40.8 m improvement vs. +0.6 m for placebo; difference +40.2 m (p<0.001)

Key secondary endpoints:

• •PVR: -234 dyn·s·cm⁻⁵ vs. placebo (significant reduction)
• •NT-proBNP: significant reduction
• •WHO functional class improvement: more patients improved 1 WHO class
• •Clinical worsening: 7% vs. 24% for placebo (significantly lower)

FDA approval: March 2024 (Winrevair, Merck). First PAH approval that targets pulmonary vascular remodeling mechanisms rather than just vasodilation. Combined with background PAH therapies (PDE5 inhibitors, endothelin receptor antagonists, prostacyclins).

Activin A in Inflammation and Infection

Acute Phase Response

Activin A is rapidly released during acute inflammation:

• •Rises within hours of LPS injection, trauma, or surgery
• •Produced by macrophages, DCs, eosinophils, and epithelial cells in response to TLR ligands
• •Activin A amplifies the inflammatory response: promotes IL-6 and TNF-α production, monocyte activation
• •Also has anti-inflammatory effects depending on context: can promote IL-10 production, regulatory T cell differentiation

This context-dependent bidirectional activity (pro- vs. anti-inflammatory) reflects cell-type and co-stimulation-specific SMAD2/3 target gene programs — similar to TGF-β's dual role in immunity.

Activin A in Allergic Inflammation

Activin A is elevated in asthma airways and promotes eosinophil survival, mast cell activation, and airway remodeling (subepithelial fibrosis). Blocking activin A in murine asthma models reduces eosinophilia and airway hyperresponsiveness.

Activin A in COVID-19

Severe COVID-19 is associated with markedly elevated serum activin A, likely from macrophage activation and cytokine storm. Activin A may contribute to the COVID-associated coagulopathy and fibrotic lung injury through SMAD2/3-driven endothelial dysfunction and myofibroblast activation.

Activin A in Bone Metabolism

Activin A inhibits bone formation:

• •Osteoblasts: activin A suppresses osteoblast differentiation via SMAD2/3 → reduces Runx2/Osterix expression
• •Osteoclasts: activin A stimulates RANKL expression → promotes osteoclastogenesis
• •Net effect: reduces bone formation, increases resorption → bone loss

Activin A is elevated in multiple myeloma bone microenvironment and contributes to osteolytic lesions and anemia. Luspatercept in myeloma was partly rationalized by activin A blockade reducing myeloma bone disease.

Sotatercept in osteoporosis: In Phase II trials for postmenopausal osteoporosis, sotatercept dose-dependently increased bone mineral density — a pleasing side effect from the PAH development program that suggests future skeletal indications.

Activin A and Activin B: Key Differences

Notably, follistatin neutralizes activin A more effectively than activin B — meaning that when follistatin rises (as in some inflammatory contexts), activin B signaling may persist even as activin A is buffered.

Follistatin: The Endogenous Ligand Trap

Follistatin (FST) and its close relative FSTL3 (follistatin-like 3) are the primary endogenous neutralizers of activin A. Follistatin:

• •Produced by pituitary, gonads, liver, endothelium, bone marrow stroma
• •Binds activin A with Kd ~10-100 pM; the complex cannot bind ActRII
• •One follistatin molecule can bind one activin dimer in a "embrace" configuration; at higher concentrations, 2:1 stoichiometry further neutralizes the ligand
• •Follistatin sequesters activin A from ActRIIA/B with much greater efficiency than from ActRIIB when heparan sulfate is present (cell-surface tethering context)
• •FST315 (heparin-binding isoform) is locally sequestered near cell surfaces; FST288 circulates
• •Follistatin is elevated in PAH — suggesting a compensatory response to excess activin A that is insufficient to fully normalize the vascular phenotype

Research Tools and Models

Current Research Frontiers

Sotatercept in Other Pulmonary Vascular Diseases

Following PAH approval, sotatercept is being evaluated in:

• •Group 3 PAH (PH due to lung disease): SOTERIA trial
• •Group 2 PAH (PH due to left heart disease): separate design needed
• •CTEPH (chronic thromboembolic PH): potential application
• •Combination with emerging BMPR2 pathway restorers (BMP9 analogs)

Activin A as a Therapeutic Target in Fibrodysplasia Ossificans Progressiva (FOP)

FOP is caused by gain-of-function mutations in ACVR1 (ALK2), which makes ALK2 hyper-responsive to activin A (normally activin A does not signal through ALK2; the FOP mutation allows it to, triggering ectopic bone formation). Garetosmab (anti-activin A monoclonal antibody, Regeneron) and saracatinib/vixarelimab (targeting activin A signaling) are in clinical trials for FOP — directly targeting activin A rather than ALK2.

Activin A in Cancer Cachexia

Activin A is elevated in cancer cachexia and promotes skeletal muscle wasting via ActRIIB/ALK4/SMAD2/3 → FoxO activation → ubiquitin-proteasome-mediated muscle protein degradation. Bimagrumab (anti-ActRIIA/IIB) and other ActRIIB-blocking strategies are being tested in cancer cachexia and sarcopenia.

Activin A and Reproductive Pathology

Activin A dysregulation contributes to PCOS (elevated activin A → FSH dysregulation), preeclampsia (elevated maternal serum activin A in first trimester predicts preeclampsia), and endometriosis. Follistatin/activin A ratio as a biomarker is being evaluated in these contexts.

Activin A in Neuroinflammation and Neurodegeneration

Activin A is produced by astrocytes and microglia in response to CNS injury. It has neuroprotective effects in stroke and TBI models but pro-inflammatory effects in neuroinflammatory conditions. The SMAD2/3 pathway in CNS may represent a therapeutic opportunity in neurodegeneration.

Conclusion

Activin A and Activin B represent a compelling case study in how a discovery rooted in reproductive endocrinology expanded — through careful pathway analysis and serendipitous clinical observation — into treatments for two entirely different conditions: anemia (luspatercept) and pulmonary arterial hypertension (sotatercept). The conceptual insight was recognizing that SMAD2/3 signaling, whether in erythroblasts or pulmonary vascular cells, can be pathologically amplified and that soluble ActRII-Fc traps can rebalance the SMAD1/5/8 vs. SMAD2/3 axis.

For researchers, the activin system offers multiple tractable nodes: receptor-level traps (sotatercept, luspatercept), ligand-specific antibodies (garetosmab for FOP), kinase inhibitors (SB431542, A83-01 for mechanistic studies), and genetic models (Inhba/Inhbb knockouts, ActRII-conditional mice). The expanding therapeutic landscape — FOP, cachexia, osteoporosis, CTEPH, COVID lung injury — ensures that activin biology will remain at the forefront of translational research for years to come.

Key Research Citations

2. Vale W, et al. (1986). Purification and characterization of an FSH releasing protein from porcine ovarian follicular fluid. Nature, 321(6072), 776-779. PMID: 3012369

3. Massagué J (2012). TGFβ signalling in context. Nature Reviews Molecular Cell Biology, 13(10), 616-630. PMID: 22992590

4. Suragani RN, et al. (2014). Transforming growth factor-β superfamily ligand trap ACE-536 corrects anemia by promoting late-stage erythropoiesis. Nature Medicine, 20(4), 408-414. PMID: 24658078

7. Upton PD, et al. (2009). Bone morphogenetic protein (BMP) and activin type II receptors balance BMP9 signals mediated by activin receptor-like kinase-1 in human pulmonary artery endothelial cells. Journal of Biological Chemistry, 284(23), 15821-15830. PMID: 19366699

9. Olsen OE, et al. (2015). Activin A inhibits BMP-signaling by binding ACVR2A and ACVR2B. Cell Communication and Signaling, 13(1), 27. PMID: 26047946

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This article is intended for Research Use Only (RUO). Activin-related research tools and compounds described herein, including sotatercept and luspatercept analogs, are for research applications only unless specifically approved for clinical use. All studies involving activin signaling modulation must comply with applicable institutional and regulatory guidelines. This content does not constitute medical advice.